[Boatanchors] TS-830S Screen Grid Switch Issue

[w0ng at adelphia.net]

I'd greatly appreciate it if someone reading this posting might have an idea they would pass on to me as to why an otherwise good working Kenwood TS-830S transceiver literally goes "dark" when the screen grid switch is turned to the OFF position. 
 
I was replacing a mismatched pair of finals (one 6146A and one 6146W) with a good pair of 6146B's. Started to begin the neutralization procedure by loading the xcvr up on 28.500 like normal but when I turned OFF the screen grid switch on the back panel the frequency display went dark and the SEND switch became inoperative (no relay action) along with all meter functions and controls. 
 
Turned the screen grid switch back ON and everything returned to normal. Turned it OFF and again the display went dark, SEND switch was again inoperative along with everything else but the fan and indicator lamps. There was no measurable rf output from the driver stage on my scope when the screen grid switch was in the off position.
 
Really appreciate some help here as to why this is happening and how to fix it. If I haven't given enough background info, etc. let me know. Thanks in advance. Bill, w0ng at adelphia.net